1. Technical Field
The present invention relates to a temperature increasing system for vehicle batteries.
2. Related Art
In recent years, in known electric vehicles (EVs) and hybrid electric vehicles (HEVs), drive motors have been provided as drive sources for driving drive wheels of the vehicles. In such a vehicle, a battery that can supply electric power to a drive motor is installed, and the battery is charged with electric power to be supplied to the drive motor. The performance of the battery depends on temperature and is more likely to be degraded at low temperature than at normal temperature. Specifically, in accordance with a decrease in the temperature of the battery, the internal resistance of the battery is increased. Thus, the capacity and output can be decreased. Accordingly, in order to prevent degradation in the performance of the battery to be installed in the vehicle, a technique to warm up the battery has been proposed.
For example, Japanese Unexamined Patent Application Publication (JP-A) No. 2010-093883 discloses a technique for performing temperature increasing control to increase the temperature by periodically repeating charging and discharging of the battery when the temperature of the battery detected by a temperature sensor is lower than a predetermined temperature. According to this temperature increasing control, Joule heat is generated in the battery by using the internal resistance of the battery, thereby increasing the temperature of the battery.
In an extremely cold environment such as in a cold area or the like, while a vehicle is parked, the temperature of a battery may fall below the minimum operating temperature. In such a case, the battery is not operable. Accordingly, before the vehicle starts moving, the battery needs to be warmed up so that the temperature of the battery becomes higher than or equal to the minimum operating temperature. Now, in the technique disclosed in JP-A No. 2010-093883, for example, the battery is controlled to be operated when increasing the temperature of the battery. Thus, if the temperature of the battery falls below the minimum operating temperature while the vehicle is parked, it is difficult to warm up the battery before the vehicle starts moving. It is possible to maintain the temperature of the battery to be higher than or equal to the minimum operating temperature by continuously performing temperature increasing control on the battery while the vehicle is parked in a cold environment. However, since the electric power of the battery may be run out, such temperature increasing control is difficult.
The present invention has been made in view of the above. It is desirable to provide a temperature increasing system for vehicle batteries that can more reliably warm up a battery that can supply electric power to a drive motor in a cold environment.